I agree with rabbit. (Well I don't have personal experience, soon I will though, but with a GTS 250 in previous hackintosh installation all went well with just graphicsenabler=yes)
It looks like the 2 guys in insanelymac have a somewhat viable solution working (although both approach it as a concept.. rather than a full on developing idea, i hope chameleon devs pick up on this or some other team maybe a joint task OPERATION?!), it was achieved via dsdt or boot. I think the method is same just two different files handling it.
Even with 10.6.5 update, I have a feeling that the 5770 will have a hard time having flawless integration like NVIDIA Cards (well certain nvidia cards, but personally, I have dealt with 9400GT, 8600GT, GTS 250 and they all worked fine. And I will be dealing with a GTX260 either this saturday or sometime next week). IDK Something there is soemthing about ATI cards in hackintoshes.